The invention relates to a thermal image exposure plate which, via a physical transfer or conversion effect from the thermal radiation corresponding to a thermal image and incident with topical distribution pattern, generates a corresponding image of a topically distributed property such as, for example, a charge image via a pyroelectric effect, i.e., an image with topically distributed electrical charge. A conversion layer is provided which is designed with a grid formation by means of interruptions in the material. This prevents lateral heat conduction and the lack of image definition connected therewith.
In the most general sense, such a thermal image exposure plate serves as the receiver of a thermal image which is imaged onto the plate via an optical device and generates there a corresponding topically distributed temperature relief pattern due to absorption and heating. As an essential component, the thermal image exposure plate has a conversion or transfer layer as the carrier of the temperature relief pattern, said conversion layer converting or translating the temperature relief into a relief of utilizable properties. Depending upon the material employed, that can be a topically distributed charge relief pattern via the pyroelectric effect, but can also be a voltage relief pattern relative to a basic electrode via a thermal electric effect, or an image of topically distributed electric resistance via a thermal-resistive effect. Electrical values are not the only thing conceivable as the image content. Other designs are also possible, for example, a topical distribution pattern of the optical index of refraction generated by the thermal radiation. One may select the suitable effect depending on the usefulness favorable for the respective employment.
Apart from a pure conversion of the thermal image into an image visible to the naked eye, the most common use of the image generated is to scan it via a concentrated electron beam in a camera tube which generates from the image an electrical signal correspondingly occurring in timed fashion. This is either stored or displayed as a visible image via a picture tube.
The building-up of the temperature relief pattern due to the thermal radiation imaged on the thermal image exposure plate ensues according to an exponential function. On the one hand, sufficient time must be given so that the temperature relief can build up in the transfer layer, rendering possible a sufficient intensity of the image generated by the transfer layer. On the other hand, the image sharpness is determined by the lateral thermal conductivity of the transfer layer or, respectively, of the applied layers also present. The topical temperature differences built up, in accordance with the lateral thermal conductivity, have the tendency to flow into one another and, thus reduce the image sharpness.
For that reason, the incident thermal radiation is generally chopped, i.e., the thermal image is periodically faded in and out via a modulating diaphragm and, by so doing, the temperature relief is repeatedly re-built-up with the modulation frequency. Thereby, the optical resolution increases with the modulation frequency, but the sensitivity decreases due to the thermal retention time thereby becoming smaller.
In order to obtain the longest possible thermal retention time and, thus high signal sensitivity given high image resolution at the same time with low modulation frequencies or also, for example, given slow panning of the thermal image exposure device over the scene of the exposure, it is known and described, for example, in German OS No. 22 23 288 corresponding to British Pat. No. 1,395,741, to arrange the pyroelectric transfer or conversion layer of a pyroelectric camera tube in a grid. The transfer or conversion layer then consists of a mosaic of pyroelectric elements which are secured separated from one another to a carrier with low lateral thermal conductivity. By so doing, the cross-talk of the individual image points due to lateral thermal conduction is reduced. The carrier side not in a grid pattern is provided with an electrode layer permeable to thermal radiation and faces the thermal radiation side. The grid side of the transfer layer which remains open faces the scanning electron beam. For reasons of thermal conduction, the channels separating the individual mosaic points must have sufficient width, but can add nothing to the signal generation with the surface they take up. Since the lattice constant of the grid should be as small as possible in order to achieve a high image resolution, i.e., the mosaic should be as fine as possible, the ratio of the active transfer layer surface to the passive channel surface is unfavorable for a good signal sensitivity. Further, providing a carrier layer with poor lateral thermal conductivity produces a layer with relatively high thermal capacity, which likewise weakens the signal sensitivity.